1. Field of the Invention
The present invention generally relates to a method of forming a semiconductor device, a method of forming a circuit board, and a semiconductor-device forming device. The present invention particularly relates to a method of forming a semiconductor device having solder balls thereof arranged at small holes formed in a board, and a method of forming a circuit board used in such a semiconductor device, and a semiconductor-device forming device for manufacturing such a semiconductor device.
In recent years, a fine-pitch BGA (ball-grid array) has been widely used to respond to the demand for miniaturization and increased circuit density of semiconductor devices.
The fine-pitch BGA includes a semiconductor chip mounted on a surface of a board and a resin package to cover the semiconductor chip, and further includes solder balls provided as external connection terminals on the other surface of the board.
In order to further pursue miniaturization and increased circuit density of the semiconductor device, pitches between the solder balls need to be further narrowed. Since reliable semiconductor devices are expected, a certain degree of reliability must be maintained even when pitches of the solder balls are further narrowed.
2. Description of the Related Art
FIG. 1 is an illustrative drawing showing a semiconductor device 1A having a FBGA (fine-pitch ball-grid array) structure. The semiconductor device 1A of FIG. 1 is an xe2x80x9covermoldxe2x80x9d type. The semiconductor device 1A mainly includes a board 2, a semiconductor chip 3, a resin package 8, and a solder ball 10.
The board 2 is formed from a resin film, and has the semiconductor chip 3 mounted thereon via an adhesive. The board 2 has a hole 7 formed at a predetermined location thereof. A conductive sheet 5 is formed by plating copper (Cu) or gold (Au) at the hole 7 on the side where the semiconductor chip 3 is mounted. The conductive sheet 5 serves as an electrode, and is hereinafter referred to as an electrode sheet 5. In this configuration, one end of the hole 7 is closed by the electrode sheet 5.
In the hole 7, a via portion 9 is formed by using solder. The via portion 9 is connected to the solder ball 10 to together form a single inseparable part. In this manner, the solder ball 10 is electrically connected to the electrode sheet 5 through the via portion 9. The solder ball 10 serves as an external connection terminal, and is provided to project from the surface of the board 2.
In the semiconductor device 1A of the overmold type as shown in FIG. 1, the semiconductor chip 3 and the electrode sheet 5 are connected by a wire 6. The resin package 8 is formed by using a transfer mold method, for example, and serves to protect the semiconductor chip 3, the electrode sheet 5, and the wire 6.
FIG. 2 is an illustrative drawing showing a semiconductor device 1B having a FBGA structure of a flip-chip type. In the semiconductor device 1B of FIG. 2, a stud bump 11 is formed on the semiconductor chip 3, and is connected to the electrode sheet 5 via flip-chip bonding. In some configurations, a solder bump is used in place of the stud bump 11. In FIG. 2, the same elements as those of FIG. 1 are referred to by the same numerals.
The semiconductor device 1A and the semiconductor device 1B having the FBGA structure described above have the solder ball 10 serving as an external connection terminal. A manufacturing process for forming the semiconductor device 1A or the semiconductor device 1B thus necessarily includes a ball mounting step for mounting the solder ball 10 on the board 2.
FIGS. 3 through 5 are illustrative drawings showing related-art methods of mounting the solder ball 10 on the board 2. It should be noted that the methods shown in FIGS. 3 through 5 are directed to the semiconductor device 1A of FIG. l.
In FIG. 3, the solder ball 10 with a flux 12 (or solder paste) applied thereto in advance is inserted into the hole 7 of the board 2. FIG. 4 shows the way the solder ball 10 is inserted into the hole 7.
In the related art, it is possible for adjacent solder balls to have as large a pitch as 0.8 mm therebetween, so that a diameter L1 of the hole 7 can be proportionally large (e.g., can be 0.30 to 0.40 mm). In such a case, a diameter R of the solder ball 10 may generally range from 0.40 mm to 0.50 mm. When the solder ball 10 is inserted into the hole 7, the solder ball 10 may be completely buried in the hole 7, or may be partially but almost entirely cloistered in the hole 7, depending on the diameter R of the solder ball 10.
After the solder ball 10 is inserted into the hole 7, a reflow process (i.e., heating process) is performed to melt the solder ball 10. Since the solder ball 10 is completely or almost entirely cloistered in the hole 7, the melted solder ball 10 fills the hole 7 securely so as to contact the electrode sheet 5. Solder in excess of the volume of the hole 7 forms the solder ball 10 on the board 2 with help of the surface tension. In this manner, the semiconductor device 1A shown in FIG. 1 is created.
FIG. 5 shows another ball mounting method. In this method, the solder paste 13 is provided in the hole 7 by applying a printing method (i.e., a screen printing method) to the board 2. As described above, the diameter L1 of the hole 7 is relatively large in the relate-art configuration, so that the screen printing easily fills the hole 7 with the solder paste 13. Here, the solder paste 13 is a mixture of organic flux and solder powder.
The solder ball 10 is inserted into the hole 7 filled with the solder paste 13, and a reflow process is performed. This disperses organic components from the solder paste 13, and the solder powder is melted to fill the hole 7. The solder ball 10 is also melted so as to blend with the solder in the hole 7. In this manner, the semiconductor device 1A shown in FIG. 1 is created.
As a circuit density of the semiconductor chip 3 is increased, the number of external terminals tends to increase as has been observed in recent years. Also, semiconductor devices are expected to be increasingly smaller in order to produce an ever smaller electronics equipment.
Against this background, pitches between balls in semiconductor devices are now required to be as small as 0.5 mm. In order to achieve this dimension, a diameter L1 of a hole needs to be as small as 0.20 to 0.25 mm, and a diameter of a solder ball needs to be about 0.3 mm.
If the ball mounting method as described in connection with FIGS. 3 and 4 is used in such a small-dimension configuration as described above, an attempt to insert the solder ball 10 into the hole 7 ends up having the solder ball 10 only partially cloistered in the hole 7 because of the relatively small size of the hole 7 compared to the size of the solder ball 10. This creates a large gap between the solder ball 10 and the electrode sheet 5. Because of the size of the gap, the reflow process may not be able to electrically connect the solder ball 10 to the electrode sheet 5.
FIGS. 6A and 6B are illustrative drawings showing a case in which the ball mounting method of FIG. 5 is applied to the board 2 having a hole 14 with a diameter L2 of 0.20 mm. As shown in FIG. 6A, an attempt to insert the solder paste 13 in the hole 14 by using a screen printing method fails to sufficiently fill the hole 14 with the solder paste 13 when the diameter L2 of the hole 14 is as small as 0.20 mm to 0.25 mm. Namely, as shown in the figure, the solder paste 13 may be provided only around the end of the hole 14.
When the solder ball 10 is mounted in the hole 14 and a reflow process is then performed, solder of the solder paste 13 is absorbed by the melted solder ball 10, resulting in such a situation as no solder exists inside the hole 14 as shown in FIG. 6B. In this manner, the ball mounting method of FIG. 5 cannot be applied to the board 2 if the hole 14 has a small diameter since the solder ball 10 cannot be appropriately mounted in such a small hole.
Accordingly, there is a need for a method of forming a semiconductor device, a method of forming a circuit board, and a semiconductor-device forming device which can mount solder balls reliably on a board even when a diameter of holes is decreased to shorten pitches between balls.
Accordingly, it is a general object of the present invention to provide a method of forming a semiconductor device, a method of forming a circuit board, and a semiconductor-device forming device which can satisfy the need described above.
It is another and more specific object of the present invention to provide a method of forming a semiconductor device, a method of forming a circuit board, and a semiconductor-device forming device which can mount solder balls reliably on a board even when a diameter of holes is decreased to shorten pitches between balls.
In order to achieve the above objects according to the present invention, a method of forming a semiconductor device by mounting solder balls on a resin board which has holes formed therethrough and conductive sheets formed therebeneath to cover bottom ends of the holes includes the steps of applying solder paste on the holes, melting the solder paste by heat to make solder of the solder paste flow into the holes and establish contact with the conductive sheets, and connecting the solder balls to the solder filled in the holes.
According to the method described above, the solder paste is printed on the holes, and is melted to let the solder of the solder paste flow into the holes. Since the solder is in a fluid state, the solder securely fills the holes even if the holes have a small diameter such as between 0.2 mm and 0.25 mm. By the same reason, the solder filled in the holes establishes secure electrical connection with the conductive sheet.
Accordingly, when the solder balls are connected to the solder in the holes, secure connection is insured therebetween. This achieves reliable electrical connection between the solder balls and the conductive sheets. In this manner, reliable solder-ball mounting is performed even when the diameter of the holes is small.
According to one aspect of the present invention, the steps of applying the solder paste and melting the solder paste are repeated several times to insure that the solder completely fills the holes.
Further, a method of forming a semiconductor device by mounting solder balls on a resin board which has holes formed therethrough and conductive sheets formed therebeneath to cover bottom ends of the holes includes the steps of applying solder paste on the holes, and melting the solder paste by heat to make solder of the solder paste flow into the holes and establish contact with the conductive sheets and to make the solder of the solder paste in excess of a volume of each of the holes form the solder balls projecting from the resin board.
According to the method described above, the solder paste is applied on the holes in such an amount that the solder of the solder paste exceeds the volume of the holes. The solder in excess of the volume of the holes forms a ball shape due to surface tension, and the ball created in this fashion serves as an external connection terminal.
This allows a single heating step to simultaneously create the hole fillings and the solder balls, thereby achieving an efficient ball mounting process.
Moreover, a method of forming a circuit board by filling metal in holes formed through the circuit board which has conductive sheets formed therebeneath to cover bottom ends of the holes includes the steps of applying solder paste on the holes, and melting the solder paste by heat to make solder of the solder paste flow into the holes and establish contact with the conductive sheets.
The method described above forms the circuit board offering reliable electrical connections by the same reasons previously described even when the diameter of the holes is small.
According to one aspect of the present invention, the step of melting the solder pastes in all the methods described above is performed with respect to the holes that have an exposed resin surface as an inside wall thereof. Since there is no good familiarity between the solder and the resin surface in terms of surface contact, the solder flowing into the holes goes down all the way to the conductive sheets without being stuck to the inside wall halfway through the hole. This prevents generation of a void inside the holes.
Furthermore, a method of forming a circuit board by filling metal in holes formed through the circuit board comprising filling the metal in the holes by a plating process. This method further includes a step of forming a conductive sheet on the circuit board by the plating process at the same time as filling the metal in the holes.
In the method described above, the step of filling the metal in the holes is simultaneously performed with the step of forming the conductive sheets (wiring pattern). This simplifies the process of forming the circuit board.
Moreover, a device for forming a semiconductor device by mounting solder balls on a resin board which has holes formed therethrough and conductive sheets formed therebeneath to cover bottom ends of the holes includes a paste printing unit configured to apply solder paste on the holes, a heating unit configured to melt the solder paste by heat to make solder of the solder paste flow into the holes and establish contact with the conductive sheets, and a ball mounting unit configured to connect the solder balls to the solder filled in the holes.
The device as described above can practice the method of efficiently forming the semiconductor device as previously described.